Offence unsettling the heart of America? (5)
I believe the answer is:
crime
'offence' is the definition.
(thesaurus)
'unsettling the heart of america?' is the wordplay.
'unsettling' is an anagram indicator.
'the heart of' means to look at the middle letters.
The middle of 'america' is 'meric'.
'meric' with letters rearranged gives 'CRIME'.
(Other definitions for crime that I've seen before include "Speeding, perhaps" , "Transgression of the law" , "Social problem" , "Lawbreaking" , "An act punishable by law" .)